This invention relates to a timing extraction circuit, which extracts clock components, i.e., a timing signal for data regeneration, from a digital data signal and can be used for, for instance, a receiving section of a repeater for digital transmission.
The digital data signal includes a series of bits and is represented as a pulse signal of line code, e.g., a return-to-zero (RZ) line code or non-return-to-zero (NRZ) line code. FIG. 1 shows a prior art receiving section of a repeater used for digital transmission. The digital data signal received by the receiver has been distorted during transmission. A reshaping circuit 100 reshapes the distorted pulses of the digital data signal into raised cosine pulses through equalizing amplification. (The reshaping circuit 100 may descramble the digital data signal, if the signal has been scrambled.) The digital data signal having passed through the reshaping circuit 100, is fed to a timing extractor 102 having a tank circuit. The timing extractor 102 supplies a sinewave signal with a frequency corresponding to the average frequency of the digital data signal to a phase shifter 104. The phase shifter 104 includes a delay circuit and a pulser. The phase shifter 104 phase-shifts the signal from the timing extractor 102, producing a pulse signal synchronized with the shifted signal. The pulse signal is fed as a timing signal to a regeneration circuit 106 and also to a transmitting section of the repeater. The regeneration circuit 106 samples data from the digital data signal supplied from the reshaping circuit 100 in response to the leading or trailing edge of the timing signal pulses. The sampled data is fed to the transmitting section.
With the above repeater, however, in order for the timing extractor 102 to be able to obtain a sine wave signal having a frequency corresponding to the average frequency of the digital data signal, it is necessary to adjust an inductor or capacitor in the tank circuit correspondingly. In addition, if the frequency of the digital data signal is greatly deviated from the resonant frequency of the tank circuit, it must be adjusted again. Further, the output signal of the timing extractor 102 is deviated in phase even if its frequency is made equal to the frequency of the digital data signal. The phase shifter 104 is therefore necessary for obtaining a timing signal for data sampling. Moreover, the conventional timing extractor with a tank circuit is susceptible to changes in ambient conditions such as ambient temperature, so that it requires a compensation circuit.